1. Field of the Invention
This invention relates to acousto-optic tunable filters, especially those used for laser tuning.
2. Description of the prior art
Most types of acousto-optic tunable filter operate using the anisotropic acousto-optic interaction in an optically birefringent and uniaxial medium. This is the interaction between shear (transverse) acoustic waves and an incident optical wave, whereby a diffracted output optical wave has a different polarisation from that of the incident optical wave.
Our copending UK patent application no. 8718413 (U.K. Patent Application No. GB 2 207799A, published Feb. 8, 1989) discloses a tunable laser system comprising a laser cavity containing a laser amplifying element and an acousto-optic tunable filter of the type described above, the tunable filter consisting of a Bragg cell connected to a piezo-electric crystal. The piezo-electric crystal is supplied with an electrical signal from a noise generator by way of a variable frequency notch filter. The tunable filter causes Bragg diffraction of all the optical frequencies emitted from the laser amplifying element except light at the frequency required, which is allowed to pass undeflected and to return to the laser amplifying element.